Investigations of chaotic particle transport by drift waves propagating in the edge plasma of tokamaks with poloidal zonal flow are described. For large aspect ratio tokamaks, the influence of radial electric field profiles on convective cells and transport barriers, created by the nonlinear interaction between the poloidal flow and resonant waves, is investigated. For equilibria with edge shear flow, particle transport is seen to be reduced when the electric field shear is reversed. The transport reduction is attributed to the robust invariant tori that occur in nontwist Hamiltonian systems. This mechanism is proposed as an explanation for the transport reduction in Tokamak Chauffage Alfvén Brésilien [R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)] for discharges with a biased electrode at the plasma edge.

Abstract.Ion temperature gradient (ITG) and trapped electron modes (TEM) are two important micro-instabilities in the plasma core region of fusion devices (r/a ≤ 0.9). They usually coexist in the same range of spatial scale (around 0.1 < k ⊥ ρ i < 1), which makes their discrimination difficult. To investigate them, one can perform gyrokinetic simulations, transport analysis and phase velocity estimations. In Tore Supra, the identification of trapped electron modes (TEM) is made possible due to measured frequency fluctuation spectra. Indeed, turbulent spectra generally expected to be broad-band can become narrow in case of TEM turbulence, inducing "quasi-coherent" (QC) modes named QC-TEM. Therefore the analysis of frequency fluctuation spectra becomes a possible tool to differentiate TEM from ITG. We have found indications that the TEM can have a QC signature by comparing frequency fluctuation spectra from reflectometry measurements, gyrokinetic simulations and synthetic diagnostic results. Then the scope of the analysis of QC-TEM are discussed and an application is shown, namely transitions between TEM turbulence and MHD fluctuations.

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Using a voltage biased electrode inside the edge of the tokamak TCABR, H-mode discharges with strong, Regime I, and partial or total suppressed MHD activity, Regime II, were obtained. In this paper we present the results of the study of these two regimes of operation. The experiment was carried out adjusting the tokamak parameters to obtain discharges with strong or weak MHD activity, without biasing in both cases. During the shots the plasma current varied to cover a range of safety factor from 2.8 up to 3.6, allowing the Hmode barrier to interact with the magnetic islands. Subsequently, the bias was applied and shots with and without MHD activity were obtained. An array of 22 Mirnov coils was used to detect the magnetic oscillations and a triple Langmuir probe to measure plasma floating potentials and ion saturation currents. The results show that the dominant modes are m=2, n=1 and m=3, n=1 for partial excitation and suppression, respectively. In both cases strong decrease of the radial electric field is detected with destruction of the transport barrier and of the H mode regime. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of H α line intensity. The explanation of the excitation and suppression processes is discussed in the text.

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